The Collaborative International Dictionary of English v.0.48:

Gas \Gas\ (g[a^]s), n.; pl. Gases (g[a^]s"[e^]z). [Invented by
   the chemist Van Helmont of Brussels, who died in 1644.]
   1. An a["e]riform fluid; -- a term used at first by chemists
      as synonymous with air, but since restricted to fluids
      supposed to be permanently elastic, as oxygen, hydrogen,
      etc., in distinction from vapors, as steam, which become
      liquid on a reduction of temperature. In present usage,
      since all of the supposed permanent gases have been
      liquified by cold and pressure, the term has resumed
      nearly its original signification, and is applied to any
      substance in the elastic or a["e]riform state.
      [1913 Webster]

   2. (Popular Usage)
      (a) A complex mixture of gases, of which the most
          important constituents are marsh gas, olefiant gas,
          and hydrogen, artificially produced by the destructive
          distillation of gas coal, or sometimes of peat, wood,
          oil, resin, etc. It gives a brilliant light when
          burned, and is the common gas used for illuminating
          purposes.
      (b) Laughing gas.
      (c) Any irrespirable a["e]riform fluid.
          [1913 Webster]

   3. same as gasoline; -- a shortened form. Also, the
      accelerator pedal of a motor vehicle; used in the term "
      step on the gas".
      [PJC]

   4. the accelerator pedal of a motor vehicle; used in the term
      " step on the gas".
      [PJC]

   5. Same as natural gas.
      [PJC]

   6. an exceptionally enjoyable event; a good time; as, The
      concert was a gas. [slang]
      [PJC]

   Note: Gas is often used adjectively or in combination; as,
         gas fitter or gasfitter; gas meter or gas-meter, etc.
         [1913 Webster]

   Air gas (Chem.), a kind of gas made by forcing air through
      some volatile hydrocarbon, as the lighter petroleums. The
      air is so saturated with combustible vapor as to be a
      convenient illuminating and heating agent.

   Gas battery (Elec.), a form of voltaic battery, in which
      gases, especially hydrogen and oxygen, are the active
      agents.

   Gas carbon, Gas coke, etc. See under Carbon, Coke,
      etc.

   Gas coal, a bituminous or hydrogenous coal yielding a high
      percentage of volatile matters, and therefore available
      for the manufacture of illuminating gas. --R. W. Raymond.

   Gas engine, an engine in which the motion of the piston is
      produced by the combustion or sudden production or
      expansion of gas; -- especially, an engine in which an
      explosive mixture of gas and air is forced into the
      working cylinder and ignited there by a gas flame or an
      electric spark.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gas engine \Gas engine\ (Mach.)
   A kind of internal-combustion engine (which see) using fixed
   gas; also, broadly, any internal-combustion engine.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gas engine
    n 1: an internal-combustion engine similar to a gasoline engine
         but using natural gas instead of gasoline vapor